DESCRIPTION: Focusing on vowel sounds from a wide variety of languages, we use a combination of acoustic measurements of natural tokens, computer simulation of preferred vowel systems, and perceptual and statistical pattern recognition experiments to test predictions of the Theory of Adaptive Dispersion (TAD) and several of its corollaries (namely, the Size Principle, the Hyper and Hypo Theory, and the Auditory Enhancement Hypothesis). TAD states that the aim of talkers is to make phonological contrasts sufficiently distinctive to promote linguistic comprehension by the listener while minimizing the articulatory effort needed to achieve this degree of distinctiveness. Among the predictions tested are these: (1) As vowel inventories increase in size, there will tend to be a greater use of various contrast-enhancing properties, including vowel inherent spectral change, an expanded vowel space, and a greater use of distinctive vowel length differences. (2) For a given level of redundancy in the communication setting, alternative sources of contrast enhancement will tend to vary inversely across dialects of the same language. (3) Contrast enhancement will be more in evidence when a vowel is part of a word that represents new rather than given information. (4) Computer simulations of preferred vowel inventories based on TAD will tend to be more accurate when the vowel tokens are dynamically specified. Because TAD and its corollaries are primarily aimed at explaining regularities of speech production rather than of speech perception, they must be supplemented by specific hypotheses that predict how speech sounds are categorized by listeners. Two such hypotheses that will be tested are: the Tonotopic Distance Hypothesis and the Linear Boundary Hypothesis. The first of these states that perceived boundary locations among vowel categories are determined by the distances between adjacent spectral peaks but that the weight of each distance cue is inversely related to the size of the distance. The second hypothesis states that perceived category boundaries between vowel stimuli tend to be linear when formant frequencies are scaled in auditorily motivated units such as Bark. These perceptual hypotheses will be extensively tested both within and across languages.